Plugging devices are common tools used in oil and/or gas wells. Plugging devices normally comprises an inner mandrel and an outer housing, where axial displacement of the inner mandrel with respect to the outer housing provides a setting operation of the plugging device. During the setting operation, a sealing device is radially expanded from a run state to a set state. The sealing device comprises a sealing element which is preventing axial fluid flow between the outer housing and the inner surface of the well pipe and slips devices which is anchoring the plugging device to the inner surface of the well pipe. Some plugging devices are denoted as packers, i.e. the inner mandrel comprises a bore with a valve, where fluid flow is allowed through the bore when the valve is open and where fluid flow is prevented through the bore when the valve is closed.
Retrievable plugging devices are designed for retrieval from the well after its use by means of a retrieval tool, while permanent plugging devices are set in the well permanently. Permanent plugging devices are normally simpler in their design, and may withstand higher temperatures and/or pressure than retrievable plugging devices. The only way to remove a permanent plugging device is to destroy it by milling or by use of explosives etc.
If a retrievable plugging device is set in a well for several years, it is a common problem that the release mechanism does not work anymore, and hence also the retrievable plugging device must be destroyed by milling in order to remove it.
Hence, millable or drillable permanent plugging devices have been developed. They have the advantages of the permanent plugging device (i.e. simple design, may withstand high pressure and/or high temperature), but are made of materials that are easier to remove by milling, i.e. they may be made of composite materials, cast iron, brass etc.
One problem which may occur during milling is that the inner mandrel is rotating together with the mill relative to the outer housing. During this rotation, there is no milling progress of the inner mandrel, and the milling tool is prevented from milling other parts of the plugging device as well. In such cases, it may be necessary to put a lot of weight on the milling bit and/or to increase the rotation rate of the milling bit, in order to “weld” the inner mandrel to the outer housing by the resulting increased temperature, and thereby prevent relative rotation. If this does not work, it may be necessary to pump cement on top of the plugging device and let that harden before trying the milling operation again.
Another problem which may occur is that during milling, the different parts of the plugging device disassembles and falls into the well. If those disassembled parts are too big, a fishing operation may be performed in order to remove them from the well. The fishing operation is simpler when for example using magnets if the disassembled parts are relatively small. Moreover, the fishing operation may not even be necessary if the disassembled parts so small that they can be pumped away from the well bore.
One solution to this problem is to provide the inner mandrel with a non-circular outer surface and the outer housing with a corresponding non-circular inner surface in order to prevent relative rotation between the inner mandrel and the outer housing. Such a solution is shown in U.S. Pat. No. 6,491,108. One disadvantage here is that it is very expensive to machine the non-circular shapes of the inner and outer housings. Moreover, it is difficult to provide reliable seals, such as o-rings, supporting sealing rings etc. for such non-circular mandrels. Hence, it is difficult to achieve certain product ratings, for example the ISO 14310:2008 V0 rating, for such plugging devices.
US 2010/0132960 describes a drillable bridge plug with a mandrel having external splines disposed on an outer surface of the mandrel, a sealing element disposed around the mandrel, an upper cone disposed around the mandrel proximate an upper end of the sealing element, and a lower cone disposed around the mandrel proximate the lower end of the sealing element. An inner surface of the lower cone comprises internal splines configured to engage the external splines.
US 2012/0255723 describes a slip for use in the anchoring of a downhole tool in a well casing is provided. The slip's outer diameter is about equal to the inner diameter of the well casing. The slip is positioned about a mandrel and radially expands upon the application of force. The wickers on the slip deformably engage the casing in response to a force
US 2007/0246224 describes a float equipment being constructed to have the check valve in an offset and/or skewed position with respect to the centerline of the tubular housing that is part of a string. The design is applicable to poppet type check valves as well as flapper type valves that are actuated with a flow tube. The off center and/or skewed position of the valve components allows the cutting structure on a drill bit, rather than the nozzle area on the bit bottom to make intimate contact with the valve components to accelerate the milling one of the assembly and the making of additional hole beyond the recently cemented string.
The object of the invention is to provide a plugging device which can be milled out from a well in a reliable and efficient way. Hence, the object of the invention is to provide a millable plugging device where the disadvantages with prior art plugging devices are avoided. Another object is to provide that as much as possible of the plugging device is milled before the plugging device disassemble and fall into the well.